Substantially all oscilloscope current probes use Hall Effect sensors, transformers or a combination of the two to sense the magnetic field surrounding a conductor as current flows through the conductor. Such current probes output a voltage that is proportional to the sensed magnetic field while the magnetic field is proportional to the current passing through the conductor. Thus these transducers output a defined voltage per amp of current sensed.
Hall Effect sensors are used to sense DC and very tow frequency AC (<=100 kHz). Transformer-based sensors are used to sense AC only at frequencies as high as 2 GHz. Both types of probe include a ferrite core that encircles the conductor and through which the magnetic flux surrounding the conduct of passes. Since the conductor must pass through the ferrite core, there must be a loop of wire available that can be fed through the core. This style of probe has been in use for decades and has functioned well.
The explosive growth of handheld, battery powered electronic devices has generated a need for low-level current measurements so as to be able to adjust or optimize product design to improve battery life. The existing Hall Effect-or transformer-based oscilloscope probe technology is not well suited for the task of measuring milliamp and sub-milliamp current in battery powered devices.
Hall Effect elements are self-heating while measuring which results in unfavorable zero offset voltage shift as they warm up. This offset voltage shift directly effects measurement accuracy and repeatability, both of which are important when measuring small current. Transformer-only current probes cannot measure DC which is important in battery drain analysis.
The loop of wire that is necessary for the use of conventional current probes is an additional source of error. The user of these probes must install the loop of wire by breaking the circuit (such as by cutting a trace) and inserting a length of wire. Variations in the shape of the loop of wire after installation can result in measurement variations that are small but are nevertheless significant when the current being measured is small. Moreover, the loop of wire and the ferrite core it passes through create an inductor that presents a frequency-dependent impedance in series in the circuit under test that is another source of measurement error.
Finally, conventional current probes have been optimized for measuring currents of the order of 10s to 100s of amps and as such the voltage output from these probes is very small when measuring milliamps. In some cases, the voltage output is so small that it can be lost in the noise of the probe circuit of oscilloscope.
Accordingly, what is needed is a convenient ability to measure and to monitor the waveforms of currents typical of handheld portable devices over a frequency range ranging from DC to several gigahertz.